Slab amplifiers are often used to boost output of a laser system by providing extractable energy from a pumped gain medium. The large surface area of the slab allows the pump energy to be spread over a wide volume of gain material to reduce heat effects. In general, a zig-zag pattern or tightly folded resonator (TFR) design makes use of multiple beam passes through the gain material to extract energy from the pumped region of the crystal.
Referring to FIGS. 1-6, a brief overview is provided of how a laser amplifier works in accordance with the prior art. In FIGS. 1-4, a basic laser cavity 22 includes gain material 24. The cavity 22 also has at least two mirrors 26A and 26B, such as a 100% reflective mirror 26A and a 98% partially reflective mirror 26B. The cavity 22 further has a pump energy source 28, such as a laser diode or flashlamp. FIG. 1 illustrates the cavity 22 and its components in a state of non-operation. In FIG. 2, atoms in the gain material 24 receive energy from the pump source 28, which excites the electrons into higher energy states. When these electrons return to their original energy state they emit a photon. This phenomenon is called spontaneous emission of photons.
Turning now to FIG. 3, as the photons pass through the gain material 24, they also affect the atoms in the gain material 24 by stimulating them to emit more photons while in an energized state. Mirrors 26A and 26B aligned parallel to one another at each end reflect the photons back and forth, continuing this process of stimulated emission and amplification along the same beam path. Referring to FIG. 4, photons from one atom stimulate emission of photons from other atoms and the light intensity is rapidly amplified. A cascade effect occurs, and soon we have propagated many, many photons. This process is called Light Amplification by Stimulated Emission of Radiation, which is where the term “laser” comes from. As a result of one of the end mirrors 26B having less than 100% reflectivity, some of the photons are transmitted through this mirror, and this transmitted portion is the laser's output beam.
Turning now to FIG. 5, a laser cavity 22 has limitations as to how much energy can be extracted from it, depending on available pump energy from source 28, the gain material 24, and other components. So, one way more power can be obtained is to use a second stage amplifier 30. The basic second stage amplifier 30 is much like the basic laser cavity 22, except there are no mirrors to contain the photons. There is a gain material 24 and a pump source 28 that excites the material 24 into an excited state so that there are available photons being emitted. It should be noted that there can be more than one pump source 28, which can be situated on opposite sides of the gain material 24. The large surface area of the slab allows the pump energy to be spread over a wide volume of gain material 24 to reduce localized heat effects.
Referring now to FIG. 6, the laser beam from the laser cavity 22 enters the amplifier pumped gain material 24 of the second stage amplifier 30. As was the case with the laser cavity 22 itself, photons from one atom stimulate emission of photons from other atoms and the light intensity is amplified as it passes through the amplifier gain material 24. It should be readily understood that although a single straight path of the amplified beam through the gain material 24 is shown, many amplifiers make use of a zigzag path, or tightly folded resonator (TFR) through the gain material 24 to make the best use of the excited gain material 24.
One problem encountered in the scenarios described above is that a polished uncoated air/glass interface has about 4% reflectivity. This property of gain material surfaces means that the polished parallel surfaces of the amplifier gain material can act as the mirrors of a laser cavity. This parasitic oscillation thereby depletes the available gain for the beam that we intend to amplify in the first place.
Most of the current solutions to the aforementioned issue make use of anti-reflection (AR) coatings to limit the 4% reflection effect that contributes to parasitic oscillation. Some of these solutions that make use of a zigzag beam path may also employ high-reflection coatings to facilitate reflecting the beam to be amplified off of the surfaces where desired, such as low angle of incidence beam reflections inside of the gain material.